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DEVELOPMENT OF HIGH TOUGHNESS API 5L X70MS PIPE FOR
OFFSHORE ULTRA-DEEP WATER APPLICATION
R. Silva1, M. Souza
1, L. Chad
1 and M. Teixeira
2
1TenarisConfab; Gastão Vidigal Neto, 475; Pindamonhangaba, SP, 12414900, Brazil 2Petrobras; Avenida Almirante Barroso, 81; Rio de Janeiro, RJ, 20031-004, Brazil
Keywords: Sour Service, NACE, X70, Offshore, Toughness, Pipe, SSC, HIC, HAZ, Girth Weld,
Seam Weld, Welding, Mechanical Properties, Hardness, Microstructure, Charpy Toughness,
CTOD Toughness
Abstract
The exploration of oil and gas reserves of the pre-salt area at distant points from the Brazilian
coast is bringing significant challenges to the national industry. Some fields are located
approximately 250 km off the coast of Rio de Janeiro and are placed at 2500 meters depth, with
forecasts to be up to 3000 meters, where the pipe strength becomes an important challenge due to
present installation methods and due to the considerable load caused by the pipe weight resulting
from the heavy wall thickness necessary to resist the external collapse pressure. Another
important point is that, due to the hostile environment in which the new fields are placed it is
necessary to develop new products in order to meet the rigorous mechanical and corrosion
property requirements such as toughness, and corrosion resistance in the presence of H2S. The
need to produce and transport large volumes of gas under safe conditions demands the use of
large diameter steel pipes produced by the UOE-SAWL process, which is a proven option that
has already been applied in some important projects. Considering the above situation and the fact
that nowadays the maximum pipe grade available to comply with the pre-salt gas pipeline
requirements is X65, it is necessary to study and develop a higher pipe grade in order to reduce
the wall thickness without reducing HIC resistance. This work presents the evaluation of
mechanical properties and corrosion resistance in the presence of H2S for OD of 20 inches,
WT 25.4 mm APL 5L X70MS pipe. Very good Charpy and CTOD values (Charpy down
to -60 °C and CTOD down to -20 °C) have been obtained and the pipes also have exhibited
satisfactory results when submitted to HIC and SSC tests.
Introduction
Because of the constant necessity for material improvement imposed on the market by the oil
and gas industry demands, efforts have been expended by the steel and tubular products
manufacturers, in conjunction with research and development centers, to reach higher standards
and satisfy new material requests.
The main goal of such efforts is to achieve steels with high strength and high toughness at low
temperatures, complying with the American Petroleum Institute specifications [1]. Following this
worldwide tendency, the Brazilian market presented a need to qualify API 5L Grade X70MS
pipe for offshore application.
In addition to API specifications, the main offshore Brazilian projects also demanded compliance
with special requirements including DNV-OS-F101, considering that the perspective is to apply
such products in ultra-deep waters (operating depths up to 3000 meters). To make this
qualification possible, an evaluation of UOE-SAWL API X70MS grade was performed aiming
to comply with the requirements of the DNV-OS-F101 standard for sour service and additional
requirements of the client for girth welding (more stringent sour service resistance after plastic
deformation cycles). The results obtained for mechanical properties, SSC and HIC resistance are
presented in this paper for the linepipe and girth weld joints for two heat input conditions.
High strength low alloy steels have been developed complying with API requirements, and prior
testing proved that the pipe material exhibited satisfactory quality; however, the next great
challenge was its application in the field as it involves welding with low heat inputs. Welding
aspects deserve special attention because the majority of products obtained from such materials
will be applied in production plants and transportation of oil and natural gas, which implies that
these products will need to be welded to become pipelines [2]. The evaluation was focused on
pipe weldability based on: base metal, the longitudinal weld and girth welds. The latter were
produced to represent actual field welding conditions.
Furthermore, microstructural aspects regarding the influence of various alloying elements
(nickel, chromium, molybdenum, vanadium, niobium, etc.) on the mechanical properties and
SSC and HIC resistance of these steels has raised discussions and questions between engineers
and researchers [3-5,7-9].
In order to achieve the desired properties, the steel was produced using state of the art
technology in steelmaking, such as optimum rigorous alloy design, vacuum degassing, dynamic
soft reduction, etc. The effects of these techniques, associated with Thermomechanical
Controlled Processing plus Accelerated Cooling (TMCP+ACC) resulted in a higher strength and
higher toughness with excellent weldability and sour service resistance.
Experimental Program
The pipes used for this development were produced at the TenarisConfab UOE-SAWL mill
located at Pindamonhangaba City – São Paulo – Brazil. The plates were supplied by an
internationally recognized plate mill. Girth weld joints were produced at TenarisTamsa Research
and Development Center located in Veracruz City – Mexico.
Pipe Properties Evaluation
Chemical Analysis
Table I shows the minimum and maximum values obtained from chemical analyses of the pipes.
Table I. Chemical Analysis – Base Metal (wt.%)
Element C Mn S Si P N Al Ni Cu B Nb+V+Ti Cr+Mo+V Pcm
Min 0.04 1.40 0.0007 0.30 0.007 0.004 0.025 0.008 0.012 0.0002 0.055 0.105 0.14
Max 0.06 1.54 0.0008 0.35 0.008 0.005 0.029 0.016 0.015 0.004 0.060 0.188 0.17
The chemical composition of the steel complies with the requirements of DNV-OS-F101 for
DSAW 485 SFD pipe production. It can be observed that there is a very low quantity of carbon
and alloying elements. Moreover, good practices of steelmaking and rolling processes combined
with this steel composition provided refined microstructures, and were responsible for the high
mechanical properties and good SSC and HIC resistance.
Macrostructure Analysis
A macrograph was taken from a flat section of the linepipe polished and etched by an appropriate
reagent in order to analyze the appearance of the longitudinal weld with respect to requirements
described in the DNV OS F101 standard.
Figure 1 shows the macrograph of the longitudinal weld. It is possible to observe the different
areas present in the weld sample – weld metal, heat affected zone (HAZ) and base metal – as
well as the presence of any defects or indication in the weld metal. In this case, no defects were
found and the weld joint was approved.
Figure 1. Weld macrograph – Magnification: x0.7.
Microstructural Analysis
The fine grained acicular ferrite and bainite present in the base metal produced high strength
with good toughness and the low quantity of inclusions with a homogeneous microstructure
suppressed the occurrence of HIC. Figure 2 shows the microstructures present in the pipe base
and weld metal.
(a) (b)
Figure 2. Microstructures; (a) base material and (b) weld metal.
The microstructure of the weld metal was analyzed for the steel pipes presented in Figure 2. The
weld has a primary microstructure characterized by acicular ferrite with no proeutectoid ferrite
which explains the good toughness at low temperatures.
The HAZ microstructure is positively influenced by the fine microstructure obtained in the base
metal. Figures 3 and 4 show the different microstructures in the HAZ according to the distance
from the fusion line and the peak temperature reached during the welding cycle.
(a) (b)
Figure 3. HAZ microstructures; (a) coarse grain zone and (b) fine grain zone.
(a) (b)
Figure 4. HAZ microstructures; (a) subcritical zone and (b) intercritical zone.
Tensile Tests
In order to determine the mechanical properties of the linepipe, tensile tests were performed in
the base metal and longitudinal weld, in accordance with DNV-OS-F101. Test samples were
taken in the transverse direction of the linepipe at the following locations: longitudinal weld, 90°
and 180° from the weld seam. Results obtained are presented in Table II.
Table II. Mechanical Properties of Base Metal
Position YS(MPa) UTS(MPa) YS/UTS El(%)
TT 90o 520 614 0.85 53.5
546 612 0.89 53.9
TT 180o 502 599 0.84 54.7
517 620 0.83 52.8
Weld - 648 - -
- 646 - -
Guided Bend Test
For guided bend testing, specimens were taken transverse to the longitudinal weld direction. The
samples (side bend) were prepared in accordance with ASTM A 370. The tests were carried out
and there was no evidence of cracks or ruptures in the parent metal, heat affected zone and weld
metal or fusion line.
Hardness Tests
Hardness tests were performed in conformance with the ASTM E384 standard. To perform
hardness measurements, Figure (H1b) of the ISO 3183:2007(E) standard was used as a reference.
All specimens complied with the requirements of DNV-OS-F101 (250 HV 10 maximum).
Figure 5 shows the average hardness variation throughout the samples.
Figure 5. Hardness test results - longitudinal joint.
From the results presented above, it is possible to observe that values for all locations were
below 250 HV 10 (requirement for sour service conditions), including the portion of the
longitudinal weld.
Charpy V-notch Test
The Charpy V-notch tests were performed in accordance with ASTM A370. The tests were
performed at different temperatures (23 ºC, 0 ºC, -20 ºC, -40 ºC and -60 ºC) and applied in the
Base Metal, Fusion Line, Fusion line + 2 mm and Fusion Line + 5 mm. The tests were performed
using sub-size specimens (10 x 5.0 x 55 mm) due to the load restrictions of the equipment used.
It is important to state that, from the 5 specimens tested in each condition, the minimum and the
maximum values were eliminated, presenting thus three out of five values for each temperature
and location. The Charpy values and the shear area are shown in Figures 6 and 7.
Figure 6. Charpy V-notch test results.
From Figures 6 and 7 it is possible to observe that the weld exhibits a good toughness (136 J
absorbed energy at -60 °C with shear area above 70%). Regarding the HAZ, the absorbed energy
is above 200 J in all the HAZ positions tested, from room temperature down to -60 °C with
100% of shear area, showing excellent HAZ toughness.
Figure 7. Shear area results from Charpy V-notch tests.
Sub-size energy values were converted to the full size specimen energy values shown in
Figures 6 and 7 according to the equation presented in DNV OS F101.
Cracking Tip Opening Displacement – CTOD
The CTOD tests were carried out as per BS 7448 Parts 1 and 2. The tests permitted the analysis
of the base metal, longitudinal weld center line and fusion line. The tests were conducted at two
different temperatures: 0 ºC and -20 ºC. The specimen types tested were SENB (Bx2B)
specimens. The results are shown in Figures 8 and 9.
Figure 8. CTOD test results at 0 °C.
According to Figures 8 and 9, five CTOD specimens were tested at each temperature, for the
base metal, longitudinal weld metal and HAZ. The results show values higher than the minimum
specified by DNV-OS-F101 (0.20 mm), thereby confirming the good toughness behavior already
shown in the Charpy tests.
Figure 9. CTOD test results at -20 °C.
Sulfide Stress Corrosion and Hydrogen Induced Cracking Tests Results
Hydrogen Induced Cracking (HIC) tests were carried out in conformance with NACE MR0175
and NACE TM0284. The samples were taken according to the standard for DSAW linepipes and
tested using the test Solution B of NACE TM0177. After examination no cracks were detected.
Sulfide Stress Corrosion cracking (SSC) tests were carried out in conformance with NACE
MR0175 and NACE TM0177, and specimens were tested by Method A. The samples were taken
from the longitudinal weld bead and at 90° and 180° from the weld and performed using the test
Solution B of NACE TM177. During testing, no specimen rupture was found and subsequent
examination confirmed no cracks in the specimens tested.
Girth Weld Evaluation
For evaluation of the girth weld properties of this new material under sour service conditions,
two heat inputs were considered. The aim in this part of the qualification was to assess girth
welding heat affected zone properties as described in the following paragraphs.
Welding Procedures and Specification
In order to perform the characterization of the girth weld HAZ and determine its properties, a
welding heat input work window typically applied for offshore application welding processes
was used. Thus, two different heat inputs representing typically the minimum and the maximum
heat inputs of real operations were chosen.
Due to welding equipment characteristics in the field and with the focus on the heat input effect
for the pipe girth weld HAZ, it was decided to use submerged arc welding (SAW) in order to
produce the weld joints. In such a case, two small diameter wires were applied, combined with a
low hydrogen flux and appropriate current, voltage and weld speeds in order to produce the weld
joint with a minimum risk of discontinuities, mainly on the straight side of the joint (single bevel
preparation).
The low heat input chosen was between 0.7 and 0.8 kJ/mm and the high heat input was between
1.2 and 1.3 kJ/mm. It was used with an API RP 2Z bevel design for all the welding tests as
shown in Figure 10 to properly evaluate properties in the girth weld HAZ.
Figure 10. Diagram of bevel design.
The challenge during this part of the qualification was to perform welding with this groove
configuration: the major problems with this geometry were torch accessibility and cleanness
between passes in multiple pass welding. Care was taken during operation to avoid any
discontinuity and defects.
Mechanical Properties Evaluation
Mechanical tests for both heat inputs were performed in order to evaluate the properties of the
HAZ of the girth welded pipes.
Macrostructure Analysis
Macrographs were performed in order to analyze and record the appearance at both heat inputs,
of weld joints and weld metal. As expected, it should be noted that the high heat input sample
produces wider HAZs compared with the low heat input welds. Both macros show that a straight
side with a homogeneous HAZ was achieved in both heat input weld joints, thus it was possible
to perform Charpy V-notch and CTOD tests in those areas of concern without interference of the
weld metal. Figure 11 shows macrographs of the weld joints for both heat inputs.
(a) (b)
Figure 11. Macro sections; (a) Low heat input (Magnification: x0.7) and (b) High heat input
(Magnification: x0.7).
Tensile Tests
For the tensile tests, 3 samples were taken from the transverse direction with respect to the girth
weld. All samples failed in the parent metal, leading to the conclusion that strength properties of
the weld and HAZ were superior when compared with the base metal (not heat affected
material).
Guided Bend Tests
For guided bend tests, two samples transverse to the direction of the girth welded joint were
tested for each heat input. Test specimens were prepared according to ASTM A 370. No cracks
or ruptures were evident in the weld metal, HAZ (especially fusion line) and parent metal (longer
than 3.2 mm) were detected, even for the straight side of the weld joints.
Hardness Tests
For each heat input, Vickers hardness traverses were also performed according to Figure 10(c)
from APPENDIX B of DNV-OS-F101. Two samples in the same welded ring were analyzed.
The results obtained for both heat inputs are shown in Figures 12 and 13.
Figure 12. Hardness results using low heat input.
From Figure 12 it is possible to observe that all base metal and heat affected zone results were
below 250 HV 10 (sour service condition maximum) using low heat input. The girth weld
exhibited some points above this threshold; however, weld metal was not the focus of this
investigation and could be later optimized through proper flux/wire selection.
Figure 13. Hardness results using high heat input.
From the results presented in Figure 13 it is possible to observe that all base metal and heat
affected zone results were below 250 HV 10 (sour service condition maximum) using high heat
input.
Charpy V-notch Tests
The Charpy V-notch test specimens were taken from different regions of the girth weld HAZ for
each heat input. Notch locations were placed in three different regions of the HAZ: Fusion Line
(FL), Fusion Line + 2 mm (FL+2) and Fusion Line + 5 mm (FL+5). For each of these regions, at
each heat input, 5 specimens were tested at the following temperatures: 27 °C,
0 °C, -20 °C, -40 °C and -60 °C.
Following DNV-OS-F101 and Petrobras special requirements, all tests were performed with full
size specimens (10 x 10 x 55 mm). The results obtained are shown in Figures 14 and 15.
Figure 14. Charpy test results from girth weld using high heat input – X70MS.
For both heat inputs and all locations, absorbed energy values were very good (minimum 126 J
at -40 °C) except at -60 °C, where specimens exhibited energy values less than 50 J at the fusion
line. This was associated with a small lack of fusion detected at the fusion line after the Charpy
test.
Figure15. Charpy test results from girth weld using low heat input – X70MS.
CTOD Tests
For CTOD tests, two sets of five specimens were tested for each heat input. The tests were
carried out at two different temperatures, 0 °C and -20 °C. The criteria established for validation
was a CTOD value greater than 0.15 mm. Tables III and IV present the CTOD values obtained.
Results were all above the minimum required.
Table III. CTOD Results Using Low Heat Input
Pipe and
Position Temp. CP
CTOD
[mm] Fracture
type Pipe and Position
Temp. CP CTOD
[mm] Fracture
type
LHI 1 1.30 δm LHI 1 0.50 δc
LS-3089 LHI 0 °C LHI 2 1.30 δm LS-3092 LHI -20 °C LHI 2 0.81 δm
LHI 3 1.29 δm LHI 3 0.20 δc
Table IV. CTOD Results Using High Heat Input
Pipe and
Position Temp. CP
CTOD
[mm] Fracture
type Pipe and
Position Temp. CP
CTOD
[mm] Fracture
type
HHI 1 0.78 δm HHI 1 0.52 δm
LS-3049 HHI 0 °C HHI 2 1.32 δm LS-3082 HHI -20 °C HHI 2 0.25 δc
HHI 3 0.44 δm HHI 3 0.24 δc
The tested CTOD geometry was Bx2B according to BS 7448 Part 2 with highest thickness
possible and the notch located at the fusion line with T-T orientation and crack plane orientation
NP.
SSC Corrosion Tests
SSC corrosion tests were performed using four point bend (FPBT) specimens and according to
the ASTM G-39 standard using test Solution A of NACE TM 0177. The testing duration was
720 hours and the stress applied was 100% of the minimum value of yield stress obtained during
the tensile tests for base material evaluation (in this case, 520 MPa). A set of 3 specimens was
tested for each heat input. Specimens were taken as close as possible to the internal surface of the
linepipe. Results obtained are shown in Tables V and VI.
Table V. Four Point Bend Test Results Using Low Heat Input
Identification Initial Values Final Values Stress Applied Results
Sample Specimen Saturation
ppm pH
Saturation
ppm pH %AYS
LS-3097
LHI
1 2520.44 2.65 2673.71 3.48 100 No Failure
2 2520.44 2.65 2673.71 3.48 100 No Failure
3 2520.44 2.65 2673.71 3.48 100 No Failure
Table VI. Four Point Bend Test Results Using High Heat Input
Identification Initial Values Final Values Stress Applied Results
Sample Specimen Saturation
ppm pH
Saturation
ppm pH %AYS
LS-3096
HHI
1 2520.44 2.65 2673.71 3.48 100 No Failure
2 2520.44 2.65 2673.71 3.48 100 No Failure
3 2520.44 2.65 2673.71 3.48 100 No Failure
The conclusion is that the HAZs of this material, welded with the investigated range of heat
inputs and tested according to the standard at 100% of actual yield strength are resistant to SSC
according to the criteria of NACE TM 0177 when tested in Solution A.
Conclusions
A complete evaluation of linepipe mechanical properties and SSC and HIC resistance was
performed for API 5L X70MS linepipe, considering base material, HAZ and longitudinal weld
joint. The results complied with strict requirements according to DNV-OSF-101,
NACE TM 0284 and NACE TM 0177 standards and well beyond special requirements imposed
by Petrobras.
In addition evaluation of linepipe girth welding, especially HAZ, for two heat inputs commonly
applied in offshore girth welding application was carried out. Mechanical properties of the HAZ
were performed and the results also comply with the strict requirements. Excellent fracture
toughness properties and SSC resistance were achieved in the girth weld HAZ areas showing the
acceptability of the product for higher requirements application.
Considering offshore installation, this work will be continued in order to study pipe and girth
weld behavior after plastic deformation and ageing.
References
1. American Petroleum Institute, ANSI/API Recommended Practice 2Z – 3rd Edition,
December, 1998: Recommended Practice for Preproduction Qualification for Steel Plates for
Offshore Structures.
2. American Petroleum Institute, ANSI/API Specification 5L/ISO 3183:2007 – 44th Edition,
October 1, 2008: Petroleum and Natural Gas Industries – Steel Pipe for Pipeline Transportation
System.
3. B. Beidokti, A.H. Koukabi and A. Dolati, “Effect of Titanium Addition on the Microstructure
and Inclusion Formation in Submerged Arc Welded HSLA Pipeline Steel,” Journal of Materials
Processing Technology, 209 (2009), 4027-4035.
4. S.D. Bhole et al., “Effect of Nickel and Molybdenum Addition on Weld Metal Toughness in a
Submerged Arc Welded HSLA Line-pipe Steel,” Journal of Materials Processing Technology,
173 (2005), 92-100.
5. S. Deshimaru et al., “Steel for Production, Transportation and Storage of Energy” (JFE
Technical Report, number 2, 2004, 55-67).
6. Det Norske Veritas, Offshore Standard DNV-OS-F101 – Submarine Pipeline Systems,
October 2007.
7. S.B. Ivani et al., “High-strength Steel Development for Pipelines: A Brazilian Perspective,”
Metallurgical and Materials Transactions A, 36A (2005), 443-454.
8. K. Ki-Bong, “Development of High-performance Steel with Excellent Weldability,”
Proceedings of the Fifteenth International Offshore and Polar Engineering Conference, The
International Society of Offshore and Polar Engineers, (2005).
9. P. Korczac, “Influence of Controlled Rolling Condition on Microstructure and Mechanical
Properties of Low Carbon Micro-alloyed Steels,” Journal of Materials Processing Technology,
157-158 (2004), 553-556.
10. NACE MR 0175/ISO 15156-1:2001: Petroleum and Natural Gas Industries – Materials for
Use in H2S-containing Environments in Oil and Gas Production – part 1: General Principles for
Selection of Cracking-resistant Materials.
11. NACE MR 0175/ISO 15156-2:2001: Petroleum and Natural Gas Industries – Materials for
Use in H2S-containing Environments in Oil and Gas Production – part 2: Cracking-resistant
Carbon and Low Alloy Steel, and the Use of Cast Irons.
12. NACE Standard TM0177-2005: Standard Method – Laboratory Testing of Metals for
Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments.
13. NACE Standard TM0284-2003: Standard Test Method – Evaluation of Pipeline and Pressure
Vessel Steels for Resistance to Hydrogen-induced Cracking.
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